Thermal sensor positioning in a microwave waveguide

ABSTRACT

A method of positioning on a microwave waveguide a sensor ( 20 ) including an elongate metallic element ( 23, 24 ) comprising: selecting a tubular waveguide ( 12 ); determining the general orientation of the magnetic field ( 3 ) generated during microwave transmission; and positioning the elongate metallic element ( 20, 23, 24 ) substantially parallel to the orientation of the magnetic field ( 3 ). Connections ( 23, 24 ) of the sensor ( 20 ) extend longitudinally of the waveguide ( 12 ) and are connected to the outer wall ( 25 ) of the waveguide and the central conductor ( 16 ) of the coaxial cable ( 15 ) that powers the waveguide.

TECHNICAL FIELD

This invention relates to positioning a sensor on a microwave device,especially an applicator for treatment of a body by means of microwaveelectromagnetic energy, and also relates to an applicator including asensor positioned thereon.

In our prior published application No. WO95/04385, the contents of whichare incorporated herein by reference, we have disclosed apparatus forthe treatment of menorrhagia which involves applying microwaveelectromagnetic energy at a frequency which will be substantiallycompletely absorbed by the endometrium, monitoring the operatingtemperature to ensure that the endometrium tissue is heated to about 60°and maintaining the application of the microwave energy for a period oftime sufficient to destroy the cells of the endometrium.

The temperature is therefore important and a temperature sensor in theform of a thermocouple is used to monitor the temperature on an ongoingbasis during application.

The problem which arises is that a thermocouple is constructed of metaland the application of microwave energy tends to cause direct heating ofthe thermocouple which leads to errors in the temperature readings. Thisgeneral problem is discussed in S. B. Field and J. W. Hand “AnIntroduction to the Practical Aspects of Clinical Hyperthermia” at pages459-465. As a result of the problems encountered with metallic sensors,it has been the practice to take readings either when the power is off,which precludes real-time measurement, or measurement has been bynon-metallic sensors, such as fibre-optic sensors, which are much moreexpensive.

Microwave electromagnetic energy can be propagated either by coaxialwaveguide or by tubular waveguide typically of circular cross-section.

DISCLOSURE OF THE INVENTION

The invention consists in a method of positioning on a microwavewaveguide a sensor including an elongate metallic element comprising:selecting a tubular waveguide; determining the general orientation ofthe magnetic field generated during microwave transmission; andpositioning the elongate metallic element substantially parallel to theorientation of the magnetic field.

With this arrangement, current should not be induced in the metallicelement by the magnetic field and there should therefore be little or nointerference with the parameter being sensed. Typically, the sensor willbe a thermocouple sensing temperature and the inherent danger isinterference by current flowing in the metal sheath of the thermocouple.

The invention also consists in a microwave applicator comprising atubular waveguide which, on transmission of microwaves, generates anelectric field orientated substantially perpendicular to the waveguidewall and a magnetic field substantially perpendicular to the electricfield, and a sensor including an elongate metallic element, saidelongate metallic substantially no current is induced in the metallicelement of the sensor which would otherwise cause distortion.

DESCRIPTION OF THE DRAWINGS

The invention wide now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic side elevation of a coaxial waveguide operatingin the TEM mode showing the electric and magnetic fields;

FIG. 2 is a diagrammatic cross-section of a circular waveguide accordingto the invention operating in the TE11 mode;

FIG. 3 is a diagrammatic plan view of the circular waveguide of FIG. 2;

FIG. 4 is a diagrammatic side view of the circular waveguide of FIG. 2showing the current flow in the walls; and

FIG. 5 is a diagrammatic side elevation of a microwave applicator aaccording to the invention.

In FIG. 1, the diagrammatic cross-section of a coaxial waveguide isshown where (1) is the centre conductor and (2) is the outer conductor.A coaxial waveguide propagates microwave energy in the TEM mode, andboth the magnetic field (3) and the electric field (4) are alwaysperpendicular to the axis (the centre conductor). Since currents (5)always flow at right angles to the magnetic field they will always flowalong the coaxial waveguide or any other metal structure which they comeinto contact with. Therefore, wherever one places a metallic temperaturesensor (6) on a coaxial derived applicator, current will flow in themetallic sensor because the sensor is perpendicular to the magneticfield. (6) on a coaxial derived applicator, current will flow in themetallic sensor because the sensor is perpendicular to the magneticfield.

In FIG. 2, a diagrammatic cross-section of a circular waveguide (7) isshown where magnetic field lines (3) and the electric field lines (4)are illustrated for the transverse electric mode TE11. In thisarrangement, the electric field is always perpendicular to the waveguidewall (8) and the magnetic field is always perpendicular to the electricfield.

FIG. 3 shows a diagrammatic top view of field distributions along thecircular waveguide (7) of FIG. 2. Magnetic field loops (3) are separatedby regions of high electric field (4). Note that the magnetic fieldloops are parallel to the sides of the waveguide wall (8).

FIG. 4 shows a diagrammatic side view of current flow in the walls ofthe circular waveguide (7) of FIG. 2. Here one can see that if ametallic sensor (6) is placed substantially parallel to the magneticfield at the side of the waveguide wall (8), then all current paths willcross the sensor and there will be no generated current flow in thesensor (6).

We have found that by placing the thermocouple sensor (6) substantiallyparallel to the magnetic field (3) at the wall of the waveguide (8),then substantially no current flows in the metallic elements of thesensor (6) and real-time temperature monitoring is possible without anysubstantial distortion.

The invention will now be further described by reference to FIG. 5,which is a diagrammatic side elevation of a microwave applicatorincluding a temperature sensing thermocouple positioned in accordancewith the present invention.

In FIG. 5, a microwave applicator (11) has a circular waveguide (12)filled with a dielectric material (13). The waveguide (12) terminatesshort of the end of the applicator (11) providing an exposed portion(14) which forms a radiating antenna tip for the microwave energy.Towards the end of the applicator remote from the radiating tip (14),there is a coaxial feed cable (15) having an inner conductor (16) whichdirectly excited the dielectric filled waveguide (12) via an in-linetransition (17). The inner conductor (16) passes to the centre of thedielectric material (13) and a lateral conductor (18) which passes fromthe central conductor through the outer waveguide wall (12) forms amicrowave break allowing the transition to cause the microwaves tolaunch into the dielectric material (13) as shown in FIGS. 1 to 3. Theconductor (18) is insulated by insulation as it passes through the outerconductor formed by the waveguide wall (12).

The sensor positioned in accordance with the invention is a thermocouple(20) located on the outside of the radiating tip (14) for sensing theoperating temperature. In accordance with the invention, thethermocouple (20) is positioned substantially parallel to theorientation of the magnetic field generated by the circular waveguide(12) when propagating microwaves, that is, along the line of the element(6) in FIGS. 2 and 4. Moreover, in order to avoid additional wiring, thethermocouple (20) is directly connected by a connection (23) to theouter conductor waveguide wall at (21) and by a connection (24) to thelateral conductor (18) at (22). The connections (23,24) extend parallelto one another in a plane through the axis of the waveguide, and the one(23) terminates at (21) and the other (24) extends outside the wall (12)as far as the perpendicular plane through (22), and then runs round thecircumference of the wall (12) to the conductor (18) at (22). RAccordingly, the thermocouple signal passes out along the same coaxialcable bringing the microwave power to the radiating tip (14).Conventional circuitry (not shown) is used to sense and extract the DCsignal.

The location of the thermocouple itself, at a position where there is noinduced current in operation, enables real-time sensing of the operatingtemperature without any substantial distortion.

Although not shown, the applicator (11) is provided with amicrowave-transparent protective coating of pTFE or other suitablematerial. The temperature sensor sensing thermocouple (20) is providedbetween the coating and the dielectric material as well as beinginsulated from the dielectric material.

What is claimed is:
 1. A microwave applicator comprising a tubularwaveguide which, on transmission of microwaves in transverseelectromagnetic modes, particularly in the TE₁₁ mode, generates anelectric field orientated substantially perpendicular to the waveguidewall and a magnetic field substantially perpendicular to the electricfield, and a sensor including at least one elongate conductive elementwherein said element is positioned on the waveguide so as to besubstantially parallel to said magnetic field during said microwavetransmission, and to be in a region in which said magnetic field inducesminimum current in said element, thereby to minimise induced heating. 2.A microwave applicator as claimed in claim 1, in which the dielectricmaterial extends from an output end of the waveguide so as to form anantenna to emit microwave radiation, the sensor being located on a sideof the antenna.
 3. A microwave applicator as claimed in claim 2, inwhich said element comprises sensor connections which extend parallel toone another, a first sensor connection being connected, to an outer wallof the waveguide, and a second sensor connection being connected to aconductor of a power input.
 4. A microwave applicator as claimed inclaim 3, in which the power input comprises a coaxial cable and saidconductor comprises an inner conductor of the coaxial cable whichextends into said dielectric material.
 5. A microwave applicator asclaimed in claim 4, in which a lateral conductor extends radially fromsaid conductor, and said second sensor connection is connected to anouter end of the lateral conductor.
 6. A microwave applicator as claimedin claim 5, in which the outer end of the lateral conductor extendsthrough an aperture in the outer wall of the waveguide and iselectrically insulated from it.
 7. A microwave applicator as claimed inclaim 5, in which said second sensor connection extends longitudinallyof the waveguide from the sensor and then circumferentially of the outerwall of the waveguide to the outer end of the lateral conductor.
 8. Amethod of positioning a sensor including an elongate conductive elementon a microwave waveguide comprising: selecting a tubular dielectricfilled waveguide; determining the general orientation of a magneticfield generated by the waveguide during microwave transmission; andpositioning the elongate conductive element substantially parallel tothe orientation of the magnetic field.
 9. A method as claimed in claim8, in which the waveguide is powered by a coaxial cable and in which theoutput of the sensor is connected to the coaxial cable.
 10. A microwaveapplicator comprising a tubular waveguide filled with dielectricmaterial extending from an output end of the waveguide to form anantenna; a power input comprising an inner conductor of a coaxial cablewhich extends into the dielectric material to generate microwaves intransverse electromagnetic modes, particularly in the TE₁₁ mode, fromwhich microwave energy is emitted, the microwaves within the waveguidegenerating an electric field orientated substantially perpendicular tothe waveguide wall and a magnetic field substantially perpendicular tothe electric field; and a sensor located on a side of the antennacomprising at least one elongate conductive element extendingsubstantially parallel to the magnetic field in a region in which saidmagnetic field induces minimum current in said element.